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United States Patent |
5,574,664
|
Feasey
|
November 12, 1996
|
Method for calibrating computer monitors used in the printing and
textile industries
Abstract
An apparatus for calibrating color settings of a computer monitor to cause
a proofed image, i.e., a prepress image, to essentially match a printed
image on a particular medium, allowing the aesthetic quality of the image
to be adjusted prior to printing, thus saving time and money. Embodiments
of the present invention comprise a first set of separate red, green and
blue monitor sensors (RGB) coupled to the computer monitor to sense a
reference image and a second set of RGB ambient sensors facing upwards to
sense ambient illumination. A set of RGB digital displays indicate
numerical values representative of the computer monitor illumination as
read by the monitor sensors and adjusted by the ambient sensors. A
predefined table is used to reference the indicated values for particular
medium and these values are used to adjust gamma values on the red green
and blue color guns of the computer monitor.
Inventors:
|
Feasey; Michael F. (701 Calle Cumbre, San Clemente, CA 92673)
|
Appl. No.:
|
502568 |
Filed:
|
July 14, 1995 |
Current U.S. Class: |
702/107; 345/904; 348/180; 348/181; 348/189; 348/191; 356/416; 358/518; 358/519; 702/104 |
Intern'l Class: |
G01K 019/00 |
Field of Search: |
348/180,181,189,191
345/150
364/571.07
358/189,482,483
356/416
|
References Cited
U.S. Patent Documents
4340905 | Jul., 1982 | Balding | 358/80.
|
5157506 | Nov., 1992 | Hannah | 358/298.
|
5293258 | Mar., 1994 | Dattilo | 358/518.
|
5325195 | Jun., 1994 | Ellis et al. | 358/189.
|
5459678 | Oct., 1995 | Feasey | 364/571.
|
Other References
Darcy, "Monitor Calibrators: a piece of color-management puzzle" MacWeek
Oct. 26, 1992 V6 No. 38 p. 32(2).
|
Primary Examiner: Voeltz; Emanuel T.
Assistant Examiner: Shah; Kamini S.
Attorney, Agent or Firm: Freilich Hornbacker Rosen
Parent Case Text
RELATED APPLICATIONS
This is a divisional of Ser. No. 08,169,516, filed Dec. 17, 1993 now U.S.
Pat. No. 5,459,678, which is a continuation-in-part of U.S. patent
applications Ser. No. 07/909,109 filed Jul. 2, 1992 and Ser. No.
08/014,364 filed Feb. 5, 1993 both abandoned, the disclosures of which are
expressly incorporated herein by reference.
Claims
I claim:
1. A method for calibrating color settings of a computer monitor to cause a
displayed image to essentially match a printed image on standard media,
comprising the steps of:
1) orienting a monitor sensor to sense red, green and blue monitor
illumination components from the face of the computer monitor;
2) orienting an ambient sensor to sense red, green and blue ambient
illumination components at the face of the computer monitor;
3) displaying calibration plaques on the face of the computer monitor;
4) sensing said monitor illumination components with said monitor sensor
and said ambient illumination components with said ambient sensor;
5) generating a set of red, green and blue display values as a function of
illumination components sensed by said monitor sensor and said ambient
sensor;
6) adjusting gamma values for red, green and blue color guns of the
computer monitor to cause said red, green and blue display values to
essentially match values in a predefined monitor calibration table
corresponding to at least one standard medium.
2. The method of claim 1 further comprising the step of infrared-filtering
said illumination components from the computer monitor and said ambient
illumination components prior to said sensing with said monitor sensor and
said ambient sensor.
3. The method of claim 1, wherein said calibration plaques comprise
patterns representative of a white point, a black point and a gray balance
and said calibration table is comprised of predefined values for each
calibration plaque for a plurality of media.
4. The method of claim 1, wherein said monitor sensor and said ambient
sensor are contained within a calibrator and said orienting steps use
suction cups integral to said calibrator to orient said sensors.
5. The method of claim 1, wherein said orienting steps additionally
comprise automatically providing power to circuitry associated with said
monitor sensor and said ambient sensor.
6. The method of claim 1, further comprising applying a print color gamut
look-up table in conjunction with said monitor calibration table.
7. The method of claim 6, further comprising:
loading an image from a document;
adjusting said image to aesthetic criteria of an observer; and
saving said image and said print color gamut look-up table to said
document.
8. The method of claim 1 further comprising the steps of:
disabling said ambient sensor by preventing ambient illumination from
reaching said ambient sensor; and
sensing ambient illumination with said monitor sensor to determine if said
ambient illumination is within a calibration range before proceeding.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to apparatus suitable for
calibrating computer monitors to reproduce color images that essentially
match said images as they will be printed on different media including
various types of papers or textiles.
With the advent of high resolution computer monitors connected to personal
computers executing desktop publishing tools, it has become possible and
desirable to use the computer monitor as a proof prior to printing, i.e.,
a prepress image. By first proofing images on a computer monitor, time and
expenses can be saved. However, various factors can make the image on the
computer monitor less useful for proofing purposes. First, computer
monitors are generally controlled by three color control signals that
represent red, green and blue (RGB) as opposed to the predominant printing
process which uses cyan, magenta, yellow, and black (CMYK). Solutions have
been offered in the prior art that provide conversions, using tables,
between RGB and CMYK representations of an image. However, these
conversions rely upon a standard computer monitor. Unfortunately, computer
monitors are not standard. Computer monitors manufactured by different
manufacturers or processes respond differently to the same RGB signals,
generally in a nonlinear manner to each color (R, G or B). Additionally,
computer monitors generally have external controls, e.g., contrast and
brightness, that effect their output. Also, an observer's perception of an
image on a computer monitor is altered dependent upon ambient
illumination. Therefore, to be useful for proofing, a method is required
to standardize a computer monitor's output so that a reproduced image will
be useful as a color reference to an observer.
SUMMARY OF THE INVENTION
The present invention is directed toward a method and apparatus for
calibrating a computer monitor in various ambient lighting conditions to
reproduce a color image such that said reproduced color image, also known
as a prepress image, can be used as a proof prior to printing on various
types of papers or textiles.
First, embodiments of the present invention filter infrared radiation from
a computer monitor that is to be calibrated since such nonvisible
radiation can disrupt readings that are intended to relate to the visible
spectrum. Second, a first set of RGB sensors, also filtered from infrared
radiation, are arranged to provide signals representative of the amount of
red, green or blue, that is present on an image displayed on the monitor.
Third, a second set of RGB sensors, e.g., pointed upwards, sense the RGB
values of the ambient illumination and are used to compensate the signals
from the first set of RGB sensors to obtain values independent of the
ambient illumination. The compensated values are preferably displayed on a
set of numerical displays, for each prime color.
The numerical displays are preferably calibrated to a standard computer
monitor's output. To obtain a standard computer monitor, a computer
monitor and a reference image on paper or textile are subjected to a
standard illumination, e.g., 30 foot candles as specified by the
Illumination Engineers Society (IES) with a transmissive color temperature
of 5,000 degrees Kelvin as specified by the American Standards Institute
PH2.30 or 7,500 degrees Kelvin as specified by the American Society for
Testing Materials ASTM D1684-61. An observer accordingly adjusts the
computer monitor to essentially match the color from a print of the
reference image, preferably utilizing a gamma adjustment to match the
nonlinear response of a printing process to that of the computer monitor.
A calibrator that embodies the present invention is then adjusted to a
desired value for the reference image.
Since, different media, e.g., paper types, respond differently to printing,
a data look-up table is preferably generated that corresponds to different
media, e.g., white paper or newsprint, that the computer monitor is
intended to represent. To generate this table, the aforementioned process
is repeated once for each media type.
In accordance with a preferred embodiment, the computer monitor calibrator
is primarily comprised of (1) a plurality of sensors coupled to the face
of a computer monitor where each sensor is comprised of an infrared
filter, a color filter and a photocell, (2) a plurality of sensors faced
upwards to sense ambient radiation where each sensor is comprised of an
infrared filter, a color filter and a photocell, and (3) a plurality of
digital displays that display a value representative of the computer
monitor sensors compensated by the ambient sensors.
In accordance with a further aspect of the preferred embodiment, a
predefined table is provided that lists a white point, a black point and a
gray balance for each prime color and for each defined media type.
Other features and advantages of the present invention should become
apparent from the following description of the presently-preferred
embodiments, taken in conjunction with the accompanying drawings, which
illustrate, by way of example, the principles of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of a calibrator in
accordance with the present invention;
FIG. 2 is a chart showing the spectral response of a cadium sulfide
photocell as used in a preferred embodiment;
FIG. 3 is a chart showing the spectral response of Wratten color filters as
used in a preferred embodiment;
FIG. 4 is a chart showing the spectral response of a CM-500N infrared
absorption filter as used in a preferred embodiment;
FIG. 5 is a front perspective view of a preferred calibrator in accordance
with the present invention;
FIG. 6 is a rear perspective view of a preferred calibrator in accordance
with the present invention;
FIG. 7 is a rear cutaway perspective view of a preferred calibrator in
accordance with the present invention;
FIG. 8 is a schematic of electronics that embody a calibrator in accordance
with the present invention;
FIG. 9 is a diagram of the calibrator in accordance with the present
invention used in conjunction with a computer monitor;
FIGS. 10 (a-c) are an examples of calibration plaques used for calibration
with the calibrator of the present invention;
FIGS. 11 (a-c) are a graphs of uncorrected, part corrected and fully
corrected red, blue and green gamma curves;
FIG. 12 is a front view of a preferred embodiment of a calibrator with a
printed calibration table;
FIG. 13 is a representation of the calibration procedure using a preferred
embodiment;
FIG. 14 is a representation of the environment for calibrating the
calibrator of the present invention.
FIG. 15 is a representation of the calibration procedure for a standard
computer monitor;
FIG. 16 is a flow chart of the calibrator calibration procedure;
FIG. 17 is a flow chart of the computer monitor ink calibration procedure;
and
FIG. 18 is a top level flow chart showing the ability of a calibrator of
the present invention to essentially match a color image between computer
monitors and a printed image.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Throughout the following detailed description, similar reference characters
refer to similar elements in all figures of the drawings.
With reference now to the drawings, and particularly to FIG. 1, there is
shown a block diagram of a preferred embodiment of a calibrator 10 in
accordance with the present invention. The calibrator 10 is used to adjust
a computer monitor 12 to reproduce a color image displayed on its face so
that it will essentially match the colors that will be reproduced when
said image is printed using a prescribed printing process. Once the
computer monitor's gamma values have been adjusted to essentially match
the color linearity of the computer monitor 12 to the printing process,
images displayed on the computer monitor 12 can be adjusted to the
aesthetic criteria of an observer and reliably used as proofs, thus saving
the time and expense of printing.
Computer monitors conventionally reproduce their images using three
electron guns which scan across a phosphor-coated screen. The three
electron guns individually activate phosphors which display the colors
red, green and blue (RGB), prime colors for this display process. However,
conventional printing processes instead apply inks to various types of
paper, e.g., white paper or newsprint, where the inks correspond to cyan,
magenta, yellow and black (CMYK). While tables can be created for
converting an RGB image to a CMYK image, use of the RGB image as a proof
relies upon a standardized RGB image. However, computer monitors made by
different manufacturers and processes may perform differently.
Additionally, the response of a monitor to RGB signals is essentially
linear in contrast to the response of media to a CMYK printing process.
Also, user-accessible adjustments, e.g., contrast and brightness, and
various ambient lighting conditions make it unlikely that the viewed image
will represent the printed image. Embodiments of the present invention
enable a user to adjust the RGB gamma curves of a computer monitor to
standardized settings compensated for ambient lighting.
As found in FIG. 1, the calibrator 10 in accordance with the present
invention is primarily comprised of a set of RGB monitor sensors 14 for
displaying the red, green and blue and intensities emitted from the
computer monitor 12, a set of RGB ambient sensors 16 for displaying the
red, green and blue intensities of ambient illumination, a compensator 18
for individually compensating signals from the monitor sensors 14 with
signals from the ambient sensors 16 and a set of RGB intensity displays 20
individually indicating compensated RGB signals from the compensator 18.
The RGB monitor sensors 14 are comprised of three individual sensors, a
first red sensor 22 sensitive to red illumination, a second green sensor
24 sensitive to green illumination and a third blue sensor 26 sensitive to
blue illumination. Each sensor uses a photocell, respectively 28, 30 and
32, to convert illumination into an electrical signal. In a preferred
embodiment, cadium sulfide photocells are used that are sensitive to
illumination throughout the visible spectrum from 400 to 700 m.mu., i.e.,
red, green and blue, as shown in FIG. 2 which shows the photocell
response. Therefore, to provide individual indications for red, green and
blue, filters are added to block wavelengths that are not desired as shown
in FIG. 3 which shows the filter response of Wratten filters used in a
preferred embodiment. A red filter 34, predominantly passing red
illumination and blocking other light, is placed in front of the first
photocell 28 making the electrical signal from the photocell 28 indicate
only red illumination. Similarly, a green filter 36 is used in conjunction
with photocell 30 and a blue filter 38 is used in conjunction with
photocell 32. However, it has been empirically determined that a color
filter-photocell combination cannot alone sufficiently determine the RGB
illuminations from a computer monitor due to the presence of variable
amounts of nonvisible, e.g., infrared, radiation. Therefore, a single
infrared filter 40 is additionally used to block radiation outside of the
visible spectrum. Alternatively, multiple infrared filters 40 are used
individually for each color sensor, respectively 22, 24, and 26. A
spectral response of the infrared filter 40, as found using a CM-500N
infrared absorption filter in a preferred embodiment, is shown in FIG. 4.
Even when the RGB output of a computer monitor is standardized, the
perceived color may differ when the computer monitor 12 is subjected to
various amounts of ambient radiation. As opposed to requiring a
standardized operating environment, embodiments of the present invention
permit calibrating the computer monitor 12 to its actual operating
environment. This compensation is accomplished by sensing the ambient RGB
illumination and compensating the indicated outputs of the RGB monitor
sensors 14 according to the sensed ambient RGB illumination using RGB
ambient sensors 16. The RGB ambient sensors 16, comprised of a red sensor
42, a green sensor 44 and a blue sensor 46, are each constructed in a
similar manner to the previously described monitor sensors 14 including a
photocell, a color filter and an infrared filter. However, the RGB ambient
sensors 16 are faced upwards to sense ambient illumination striking the
face of the computer monitor 12. Additionally, a plexiglass 040 opal 48 is
used as an outer layer of the RGB ambient sensors 16 to diffuse the
ambient illumination before it reaches the RGB ambient sensors 42, 44 and
46.
The compensator 18 individually adjusts each color signal from the monitor
sensors 14 according to the color signals from the ambient sensors 16.
Therefore, the signal from photocell 28, indicating red illumination is
compensated by the signal from the ambient red sensor 42. Similarly, the
signal from photocell 30 is compensated by the signal from the ambient
green sensor 44 and the signal from photocell 32 is compensated by the
signal from ambient blue sensor 46. This compensation is done for each
color individually according to the following equation:
Z=(b*y)-(a*x)
where Z is the compensated signal, x is the signal from the ambient sensor
16, y is the signal from the monitor sensor 14, and a and b are
empirically derived constants.
The compensated signals are converted to digital values and indicated on
the RGB intensity displays 20. The RGB intensity displays 20 are comprised
of a red display 50 displaying the compensated red intensity from the
computer monitor 12, a green display 52 displaying the compensated green
intensity from the computer monitor 12, and a blue display 54 displaying
the compensated blue intensity from the computer monitor 12. In a
preferred embodiment, the RGB displays 20 consist of three individual,
multi-digit, LCD displays. When the computer monitor 12 is adjusted,
according to a procedure described below using the values indicated on the
RGB intensity displays 20 and a predefined calibration table, a standard
computer monitor output is achieved that permits the computer monitor 12
to be used as a proofing device.
With reference now to FIG. 5, there is shown a front perspective view of a
calibrator 10 in accordance with an exemplary of the invention. The
calibrator 10 is preferably contained within a rectangular housing 56,
sized to fit on the face of the computer monitor 12. The RGB intensity
displays 20, respectively the red display 50, the green display 52 and the
blue display 54, are located on the front of the housing 56 for viewing by
a observer. As shown in its normal orientation, the RGB ambient sensors 16
are located on the top of the housing 56 and facing upwards to sense
ambient illumination. Additionally, a printed calibration table 58 is
located on the front of the housing 56 and associated with the RGB
intensity displays 20 to allow coordinated use during calibration as
described below.
Embodiments of the present invention are preferably battery powered. A
plurality of batteries are encased within the housing 56 and are
accessible through a battery compartment cover 60. As will be described
below, battery power is preferably automatically applied to electronics of
the calibrator 10 when the calibrator 10 is attached to the face of the
computer monitor 12.
With reference now to FIG. 6, there is shown a rear perspective view of the
housing 56 of a preferred embodiment of the calibrator 10. The housing 56
is coupled to suction cups 62 and 64, mounted on the rear of the housing
56 that are used to detachably couple the housing 56 to the face of the
computer monitor 12. The RGB monitor sensors 14, comprised of the red
sensor 22, the green sensor 24 and the blue sensor 26, are also located on
the rear of the housing and are contained within an eye cap 66. As shown
in this preferred embodiment, a single infrared filter 40 is associated
with the RGB monitor sensors 14. Suction cups 62 and 64 and eye cap 66 are
preferably formed from rubber or an equivalent resilient material that
will not scratch the face of the computer monitor 12. The RGB monitor
sensors 14 are located within the eye cap 66 such that when the housing is
coupled to the face of the computer monitor 12 using suction cups 62 and
64, the RGB computer monitor sensors 14 will be located proximate of the
face of the computer monitor 12, but not in physical contact.
Additionally, the eye cap 66 prevents extraneous ambient light from
reaching the RGB monitor sensors 14.
With reference now to FIG. 7, there is shown a rear cutaway perspective
view of an embodiment of the present invention showing means to
automatically provide power to the calibrator 10 when it is detachably
coupled to the computer monitor 12. Power for the calibrator 10 is
provided by a plurality of batteries 68 contained within a battery housing
70. Access to the batteries 68 is obtained through the battery compartment
cover 60. Battery voltage is switched to internal electronics using a
normally-off, resilient switch 72 that is coupled to one of the suction
cups, element 64 in a preferred embodiment, via a switch arm 74. The
switch arm 74 is coupled to the suction cup 64 such that when the suction
cup 64 is coupled to the computer monitor 12, the switch arm will depress
and activate the switch 72 and supply power to the internal electronics.
Conversely, when the calibrator 10 is removed from the computer monitor
12, resilience of the suction cup 64 and the switch 72 will cause the
switch 72 to flip to its normally-off position, removing power from the
internal electronics.
With reference now to FIG. 8, there is shown a schematic of prototype
electronics contained within the housing 56 that performs the functions
previously described in association with FIG. 1. The electronics are
constructed to permit independent adjustment of the a and b parameters for
each color, i.e., red, green and blue, via potentiometers. Once a
reference calibrator 10 is adjusted via procedures discussed below, the
initial settings of the potentiometers can be transferred to other
calibrators. Following this initial setup, fine tuning of an individual
calibrator 10, to account for variables in circuit manufacture is
conducted using a calibrated computer monitor and ambient light source or
with a calibration unit.
In the prototype electronics of FIG. 8, the batteries 68 are comprised of 8
AA batteries generating a nominal voltage of 12 volts. This voltage is
regulated by a fixed voltage regulator 76 to generate a fixed voltage of 5
volts to power the remaining circuitry. The remaining circuitry is divided
into three sections corresponding to each color channel, i.e., red, green
and blue channels. Each channel is comprised of four main sections, an
adjustable voltage regulator respectively, 78, 80, and 82, a compensated
light sensor network, respectively 84, 86 and 88, a scaling network,
respectively 90, 92 and 94, and the RGB digital displays, respectively 50,
52 and 54. Since each channel essentially performs in the same manner,
only the red channel will be discussed below.
The adjustable voltage regulator 78, comprised of a voltage regulator 96, a
potentiometer 98 and a fixed resistor 100, generates an adjustable and
isolated voltage that is coupled to the compensated light sensor network
84 comprised of the red sensor 22, sensing red monitor illumination, the
ambient red sensor 42, compensating the output of the red sensor 22, an A
potentiometer 102, a B potentiometer 104, and fixed resistors 106 and 108.
As previously discussed, the A potentiometer 102 and the B potentiometer
104 are used to adjust the amount that the ambient sensor 42 compensates
the red sensor 22 as read at voltage node 110. The voltage at voltage node
110 is scaled by the scaling network 90, comprised of a transistor 112 and
fixed resistors 114, 116, 118 and 120, to be compatible with the
sensitivity of the display 50 as read across the fixed resistor 120. The
green and blue channels perform in a similar manner, with some resistance
values modified to compensate for different sensitivities of the
photocells to other color ranges. As previously discussed, this circuit is
of a working prototype that embodies the present invention. It is expected
that one of ordinary skill in the art can envision other circuitry that
embody the aforementioned equation:
Z=(b*y)-(a*x),
all of which are considered to be within the scope of this invention.
With reference now to FIG. 9, there is shown the use of the calibrator 10
of the present invention in conjunction with a computer monitor 12. The
calibrator 10 is centrally mounted to the face of a computer monitor 12.
The calibration of a computer monitor is a function of the computer
monitor 12, its drive electronics, e.g., a video card located within a
computer 122, and ambient lighting 124 within the computer monitor's
environment. Thus, calibration of the computer monitor 12 will need to be
repeated if any of these items are altered. As previously described, the
present invention automatically compensates for ambient lighting via the
RGB ambient sensors 16. An observer 126 is located in front of the
computer monitor 12 where the observer 126 can view the calibrator 10 and
a calibration window 128 displayed on the computer monitor 12. The
observer 126, using a keyboard and/or mouse controls on the computer 122,
launches a calibration program, e.g., Knoll GAMMA, from the computer 122.
The calibration program places the calibration window 128 in one corner of
the face of the computer monitor 12 for interaction with the observer 126
and places calibration plaques 130 in the center of the screen for sensing
by the calibrator 10.
The calibration plaques 130, contained within black border 131 FIGS.
10(a-c), include a white point plaque 132, a gray balance plaque 134 and a
black point plaque 136. These plaques are used for establishing gamma
curves for adjusting the response of the red, green, and blue color guns
of the computer monitor 12. Black border 131 prevents distortion of
ambient light readings.
As shown in FIGS. 11(a-c), without calibration the gamma curve for each
color gun is linear. Unfortunately, in its uncalibrated and linear state a
color image reproduced on such a computer monitor will not match a printed
image. Thus, the gamma curve is adjusted to correct for this condition. To
provide this adjustment for each prime color as shown in FIG. 11, a white
point and a black point are used to adjust the end points of the gamma
curve and a gray balance is used to adjust the center point of the gamma
curves. These curves are further adjusted for a particular printing
process.
With reference now to FIGS. 12 and 13, the computer monitor calibration
procedure is now explained. In FIG. 12, a front view of the calibrator 10
is shown including the RGB intensity displays 20, respectively the red
display 50, the green display 52 and the blue display 54, and the
calibration table 58. The calibration table 58 is predefined using
procedures described below for specified standard media, e.g., white paper
or newsprint. Standard media is intended to represent media that are
commercially available from one or more manufacturers and reproducible
such that when the same printing process is repeated on multiple samples
of the standard media, the printed results will be appear identical to an
observer. Supplemental calibration tables for additional media can be
provided depending upon a user's requirements. Although the numbers shown
in FIG. 12 correspond to those used in a prototype of the present
invention which uses RGB intensity displays that read values that are in
the range of approximately 1700 to 2000 with a monitor brightness range of
approximately 1 to 15 foot candles at the monitor screen, these numbers
are presented in FIG. 12 for tutorial purposes only and are not intended
to represent actual values for a particular calibrator 10 and media
combination.
After mounting the calibrator 10 on the computer monitor 12, the observer
126 visually selects the medium for which the computer monitor is to
reproduce color proofs. For tutorial purposes, newsprint is selected. This
selection signifies that numerical values for each column are used that
are in line with the word "newsprint". Thus, the values associated with
this calibration are as follows:
______________________________________
R 1926 1721 1802
G 1940 1735 1867
B 1900 1735 1867
White Black Gray
Point Point Balance
______________________________________
Using controls on the calibration window 128, the white point calibration
plaque 132 is selected and displayed on the computer monitor 12 and sensed
by the calibrator 10. For the white point, the first column from the
calibration table is used. Thus, the following settings on the red display
50, the green display 52 and the blue display 54 are sought:
______________________________________
R 1926
G 1940
B 1900
______________________________________
To match these values, the red, green and blue color guns are adjusted
using controls in the calibration window 128. These adjustments are saved
and this process is similarly repeated for the black point using the black
point plaque 136 and then for the gray balance using the gray balance
plaque 134. Once this is completed nine separate calibration points,
associated with a particular process or medium for which the computer
monitor 12 has been calibrated, are collectively saved in memory of the
computer 122. This process may also be repeated for other predefined media
and saved for future use. With these saved gamma curves used in
conjunction with ink calibration tables described below, the computer
monitor 12 can accurately reproduce a color image that is useful as a
proof prior to printing for printing and textile industries.
With reference now to FIG. 14, there is shown a representation of the
environment for calibrating the calibrator 10 of the present invention.
This initial calibration is done once to set up the calibrator 10 and to
define the calibration table 58 that is used in conjunction with the
calibrator 10 to calibrate other computer monitors. A computer monitor 12
to be used as a reference, is placed within an environment that presents a
standard ambient illumination 124 of 30 foot candles as specified by the
Illumination Engineers Society (IES), normally from a fluorescent device,
preferably with a transmissive color temperature of 5,000 degrees Kelvin
as specified by the American Standards Institute PH2.30 or alternatively
7,500 degrees Kelvin as specified by the American Society for Testing
Materials ASTM D1684-61. In front of the computer monitor 12 a neutral
gray surface 138 is used to view reference print material 140 that is
subjected to the same ambient illumination 124. The goal of this
calibration is to configure the computer monitor 12 to accurately
reproduce the color of the reference print material 140 and thus it is
significant that the computer monitor 12 and the reference material 140 be
viewed in the same environment and in close proximity.
With reference now to FIG. 15, a paper/textile and printing process is
chosen and the reference print material 140 is placed on the neutral gray
surface 138. The reference print material 140 is comprised of an unprinted
area 142, representative of the white point, a gray 1.0 density plaque
144, representative of the gray balance, and a maximum print black ink
density plaque 146, representative of the black point. As previously
described in reference to the use of the calibrator, the calibration
program is launched which displays a calibration window 128 in one corner
of the face of the computer monitor 12. For generating a standard computer
monitor, the calibration program is used to match the computer monitor's
output to the reference print material 140 according to the observer 126.
For the white point, the white point plaque 132 is loaded by the
calibration program to the computer monitor 12. The white point plaque is
represented by L a b values of L100 a0 b0 or density levels of R255, G255,
B255. The observer 126 uses controls in the calibration window 128 to
adjust the computer monitor 12 to match the white point plaque 132 shown
on the computer monitor 12 to the unprinted area 142 of the reference
material 140. This procedure is similarly repeated for matching the black
point plaque 136, represented by L a b values of L0 a0 b0 or density
levels of R0, G0, B0, to the maximum print black ink density plaque 146
and the gray balance plaque 134, represented by L a b values of L64 a0 b0
or density levels of R76, G76, B76, to the gray 1.0 density plaque 144.
Once completed, nine points corresponding to the white point, gray balance
and black point for the colors red, green and blue have been chosen and
are saved to memory, e.g., RAM or disk, of the computer 122. These points
determine the gamma settings for this particular computer monitor 12
subjected to the present standard ambient illumination 124 that cause the
computer monitor 12 to perform as a standard computer monitor. While
determining these matches is subjectives it is a one-time operation that
can be accomplished by an observer 126 of ordinary skill in the art.
The calibrator 10, attached to the computer monitor 12, is now used to
determine gamma values to be placed in the calibration table 58, as read
from the RGB intensity displays 20, for the particular paper/textile and
printing process of the reference print material 140. The normal computer
monitor calibration procedure is now done as previously described with the
saved settings loaded to control the calibration window 128 and saved RGB
density and gamma settings applied to the plaques 132, 134 and 136, but
the values read from the intensity displays 20 are instead recorded for
entry into the calibration table 58. This procedure is reflected in FIG.
16, a flow chart of the calibrator calibration process. This process is
repeated for each paper/textile and printing process combination for which
the computer monitor 12 will be used as a proofing device. The values
determined by this process are stored in the calibration table 58 for
subsequent computer monitor 12 calibrations.
Additionally a one time procedure is used to determine the a and b
constants previously described in association with the equation:
Z=(b*y)-(a*x). To empirically determine these constants, the ambient
lighting 124 is varied between color temperatures of 4,550 and 5,500
degrees Kelvin and illuminations of 20 to 45 foot candles. As previously
described, the A and B potentiometers are recursively adjusted to obtain
values on the RGB intensity displays 20 which are independent of the
ambient lighting when the gamma curves are adjusted for matching the
reference print material 140 and a maximum print ink density 146 and gray
1.0 density 144, when subjected to altered ambient lighting 124.
With reference to FIG. 17, an additional one time process is the generation
of tables of settings that convert CMYK monitor color gamut values of
prepress images to that of print color gamut values. The prepress images
are generated from the conversion of RGB image values to CMYK values, and
from black and white negative or positive digitized cyan, magenta, yellow,
black (CMYK) color separation images into color CMYK image values. This
process for the generation of tables of settings is repeated for each
printing process. The computer monitor 12 is calibrated with the
calibrated calibrator 10 using predetermined values in the calibration
table 58 for a specified printing process. The selected values in the
calibration table 58 define the white point simulation of the selected
print medium, the black point selects the maximum print ink density and
color of the black and color ink combination that produces the maximum ink
density and the gray balance of the calibration table 58 defines the
neutral gray balance of the printing process. A CMYK color chart of
approximately 20,000 colors is printed to represent the printing process.
A printed image 148 of the CMYK color chart is prepared for the printing
process and a monitor display image 150 representing the values of the
printed CMYK color chart is produced with the computer 122 under control
of software, e.g., Adobe Photoshop. The computer monitor 12 with the
monitor display image 150 of the CMYK color chart and the printed image
148 of the CMYK color chart are displayed in an area of illumination of
the 5,000 degree Kelvin at a level of illumination of 30 foot candles,
being the approved specifications as previously defined, as provided by
the ambient lighting 124. An observer 126 with knowledge of the process
and CIE standard vision compares the monitor display image 150 with that
of the printed image 148 of the CMYK color chart. Upon examining and
comparing the color gamut of the computer monitor image to that of the
printed material, adjustments are made for required cyan, magenta, yellow,
red, green and blue in the computer monitor representation such that the
computer monitor image 150 will match the printed image 148. An ink color
gamut adjustment table 152 and a printing specification look-up table 154
of existing software tools, e.g., Adobe Photoshop, are loaded to the
computer monitor 12 and the CIE XYZ coordinates of the printing
specification look-up tables of the cyan, magenta, yellow, red, green,
blue areas of the CIE XYZ color space are remapped applying printing
specification adjustments that include dot gain compensation. The monitor
display image 150 of the printed color chart and prepress image is thus
changed to match the printed color chart and the printed image 148. This
process can also be applied to the generation of settings to convert RGB
values into print, e.g., CMYK values.
With reference now to FIG. 18, there is shown a top level flow chart of the
use of the calibrator 10 of the present invention. The printed image 148
represents printed images or a color chart reflecting the color gamut of a
printing process that is to be proofed prior to printing on computer
monitors 156 and 158. The computer monitors 156 and 158 can be located in
different facilities in different areas of the world. For a user of the
computer monitor 156 to proof an image that was aesthetically adjusted by
a user of the computer monitor 158 it is desirable that both images
essentially match each other and the printed image 148. It is to be
expected that without adjustments, there will not be an adequate match
between these images. To provide this match, the embodiments of the
present invention are used as previously described and summarized in
Blocks 160, 162 and 164 that reflect one-time operations associated with
embodiments of the present invention. In Block 160, a computer monitor 12
is adjusted to standardize its output to a particular paper/textile and
printing process. This standard computer monitor is used to calibrate the
calibrator 10 and to predetermine values for the calibration table 58 in
Block 162. In Block 164, the standard computer monitor is used as a
reference with gamut modification software to generate conversion tables
between the computer monitor's RGB display and the particular printing
process. The procedure of block 164 is further reflected in FIG. 17, a
flow chart of the printing color gamut calibration process. In Block 166,
the calibrated calibrator 10 using the predetermined values in the
calibration table 58 is used to calibrate computer monitors 156 and 158.
These calibrations are saved for each computer monitor and applied along
with the printing color gamut settings to display prepress images. As a
result of this process, the prepress images on computer monitors 156 and
158 will now represent a proof of the printed image 148.
In an alternative embodiment, the calibrator 10 can be integrated into the
computer monitor 12 rather than being detachably mounted as previously
described. As a consequence of this combination, the calibration of the
computer monitor 12 can be monitored more frequently.
In a next alternative embodiment, an automated procedure displays the
calibration plaques and interfaces to the calibrator 10 to directly read
the compensated red, green and blue values from the computer monitor 12
and automatically adjust the computer monitor's gamma curves accordingly
to a desired printing or multimedia process according to the calibration
table 58 stored within the memory of the computer 122. The interface
between the calibrator 10 and the computer 122 can be done via Apple
Desktop BUS (ADB) port, a wireless interface, or equivalent data
interface. When the calibrator 10 is integrated into the computer monitor
12, this embodiment allows frequent, automated adjustments of the computer
monitor 12. With a wired interface such as the ADB port, power can be
provided directly to the calibrator 10.
Embodiments of the present invention can also be used to calibrate black
and white, monochrome computer monitors. In such an environment, only one
sensor is required, e.g., the monitor red sensor 22 and the ambient red
sensor 42. By this process, computer monitors at local and remote sites
being so calibrated and ink tables loaded will display black and white
prepress images as a proof of how they will print.
Embodiments of the present invention can also be used as a step of
calibrating a color scanner for RGB gamma settings. By using a color
scanner to scan a color plaque and displaying the scanned image on the
calibrated computer monitor 12, the calibrator 10 can display values that
can be applied to adjust the scanner interface or to a color correction
program.
In another embodiment, the ambient sensors 16 are effectively disabled by
preventing ambient light from reaching the ambient sensors 16 with an 040
density opal at sufficient angle in front of the monitor sensors 14. This
embodiment permits readings of the ambient light at or where monitors are
intended for use, as described below. The monitor sensor readings at the
intended area of the face of a monitor are compared to a table or range of
suitable ambient readings such that a determination of suitability can be
made or an adjustment determined for permitting the ambient light
conditions to be modified to bring it within the range suitable for such
monitor calibration. Therefore, by such means a quick determination of
ambient light suitability can be determined without proceeding with
monitor calibration or having a monitor present.
With reference to FIG. 18, a further embodiment is shown. In this
embodiment, an image is aesthetically adjusted by the observer 126 of the
computer monitor 156. This image is saved into a document format that also
saves the printing color gamut settings of Block 164. In Block 160, the
calibrated calibrator 10 uses the predetermined values in the calibration
table 58 to calibrate computer monitors 156 and 158. These calibrations
are saved for each computer monitor. As a result of this process, the
prepress image on computer monitors 156 and 158 will now serves as proofs,
representative of the printed image 148.
Although the present invention has been described primarily for CMYK
correction for prepress applications, it should be understood that it is
also applicable for RGB monitor calibration useful for video and
multimedia applications. Those of ordinary skill in the art will
appreciate that various modifications can be made without departing from
the invention. Accordingly, the invention is defined by the following
claims.
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